The final steps in aldosterone biosynthesis are unclear. Undecided is whether 18-hydroxycorticosterone is a precursor of aldosterone or an end product. 18-Hydroxycorticosterone is secreted in close relationship to aldosterone. To get adequate information on the status of aldosterone in pregnancy, determination of more than one parameter of aldosterone seems to be necessary. Urinary excretion of free 18-hydroxycorticosterone, tetrahydroaldosterone, aldosterone-18-glucuronide, free aldosterone, and the plasma concentration of aldosterone were measured by radioimmunoassay in 16 primigravid women in the last trimester of normal pregnancy and in 13 healthy nonpregnant women. All steroids measured were significantly increased in pregnancy. The ratios of aldosterone-18-glucuronide to tetrahydroaldosterone in the two groups were not significantly different, so that significant changes in renal or hepatic aldosterone metabolism could not be demonstrated in pregnancy. When pregnant women with ankle edema (n = 7) were compared to pregnant women without edema (n = 9), no differences in steroid patterns could be found. The increased excretion of 18-hydroxycorticosterone in pregnancy confirms the state of hyperaldosteronism in normal pregnancy which is associated with an increase in biologically active, free aldosterone. Dissociation in the excretion of the two aldosterone metabolites and free aldosterone was found in three pregnant women, in whom excretion of aldosterone-18-glucuronide was increased but excretion of tetrahydroaldosterone and free aldosterone was in the normal nonpregnant range.